Kinematic Plate Models of the Neoproterozoic

Plate tectonic reconstructions traditionally use a combination of palaeomagnetic and geological data to model the changing positions of continents throughout Earth history. Plate reconstructions are particularly useful because they provide a framework for testing a range of hypotheses pertaining to climate, seawater chemistry, evolutionary patterns and the relationship between mantle and surface. During the Mesozoic and Cenozoic these are underpinned by data from the ocean basins that preserve relative plate motions, and data from hotspot chains and tomographic imaging of subducted slabs within the mantle to constrain absolute plate motions. For earlier times, neither ocean basins nor subducted slabs are preserved to assist with constructing plate models. Previously published plate models are usually built around times that have high quality palaeomagnetic data and between these times, the motion of continental crust is usually interpolated. Alternatively, regional tectonic models are developed predominantly from using geological data but without integrating the model into a global context. Additionally, until now all global plate models for the Neoproterozoic model only describe the configurations of continental blocks and do not explicitly consider the spatial and temporal evolution of plate boundaries. In this thesis, I present the first topological plate model of the Neoproterozoic that traces the dynamic evolution and interaction of tectonic plates, which encompass the entire earth. This model synthesises new geological and palaeomagnetic data, along with conclusions drawn from kinematic data to help discriminate competing continental configurations of the western area of the Neoproterozoic supercontinent, Rodinia. The thesis concludes by analysing the supercontinent cycle from 1000 to 0 Ma, by extracting the rift length, subduction zone length and perimeter-to-area ratio of continental crust to better understand the long-term evolution of our planet

The effectiveness of back belts as a control measure for occupational low back pain in a retail hardware chain

The objective of this study was to examine the effect of the mandatory introduction of back belts on the incidence, days lost and cost of occupational low back injuries resulting from manual handling in 11 retail hardware chain. The study was of a non-experimental before-and-after design with all retail employees in Western Australia being included in a retrospective cohort. The pre-intervention period extended for 21 months and included 2,265,933 work hours with 647 full-time equivalent positions, while the intervention period was 32 months for 4,411,352 hours worked and 827 full-time equivalent positions. Workers\u27 compensation claims for all occupational injuries occurring during the study period were analysed. During the intervention period there was a 14% reduction in the incidence frequency rate for all low back pain claims and a 33% reduction in those low back pain claims resulting in lost time, but neither reduction was statistically significant. During the intervention period there was a significant 69% reduction in the average days lost per low back pain claim and a 79 /o reduction in the days lost to low back pain per hours worked. The average direct cost was reduced by 77% for all low back pain claims and 74% for low back claims resulting in lost time, and 80% and 83% respectively when analysed per hours worked. The author concluded that the mandatory use of back belts significantly reduces the days lost due to, and the cost of occupational low back p3in resulting from manual handing in the workplace and provides a cost effective control measure if high compliance is maintained

Extreme variability in atmospheric oxygen levels in the late Precambrian

This is the final version. Available on open access from AAAS via the DOI in this recordData and materials availability: The datasets required to run the model and the code for NEOCARBSULF, which is constructed in MATLAB, can be accessed via the DOI: 10.5281/zenodo.6954788 or can be found at https://github.com/Alexjkrause/NEOCARBSULF.Mapping the history of atmospheric O2 during the late Precambrian is vital for evaluating potential links to the animal evolution. Ancient O2 levels are often inferred from geochemical analyses of marine sediments, leading to the assumption that the Earth experienced a stepwise increase in atmospheric O2 during the Neoproterozoic. However, the nature of this hypothesized oxygenation event remains unknown, with suggestions of a more dynamic O2 history in the oceans, and major uncertainty over any direct connection between the marine realm and atmospheric O2. Here we present a continuous quantitative reconstruction of atmospheric O2 over the last 1.5 billion years, using an isotope mass balance approach that combines bulk geochemistry and tectonic recycling rate calculations. We predict that atmospheric O2 levels during the Neoproterozoic oscillated between ~1% and ~50% PAL (Present Atmospheric Level). We conclude that there was no simple unidirectional rise in atmospheric O2 during the Neoproterozoic, and the first animals evolved against a backdrop of extreme O2 variability.Natural Environment Research Council (NERC)Royal SocietyLeverhulme TrustDeep Energy Community of the Deep Carbon ObservatoryRichard Lounsbery FoundationMSCA-I

A tectonic-rules-based mantle reference frame since 1 billion years ago - implications for supercontinent cycles and plate-mantle system evolution

Understanding the long-term evolution of Earth\u27s plate-mantle system is reliant on absolute plate motion models in a mantle reference frame, but such models are both difficult to construct and controversial. We present a tectonic-rules-based optimization approach to construct a plate motion model in a mantle reference frame covering the last billion years and use it as a constraint for mantle flow models. Our plate motion model results in net lithospheric rotation consistently below 0.25g g€Myr-1, in agreement with mantle flow models, while trench motions are confined to a relatively narrow range of -2 to +2g€cmg€yr-1 since 320g€Ma, during Pangea stability and dispersal. In contrast, the period from 600 to 320g€Ma, nicknamed the zippy tricentenary here, displays twice the trench motion scatter compared to more recent times, reflecting a predominance of short and highly mobile subduction zones. Our model supports an orthoversion evolution from Rodinia to Pangea with Pangea offset approximately 90g eastwards relative to Rodinia - this is the opposite sense of motion compared to a previous orthoversion hypothesis based on paleomagnetic data. In our coupled plate-mantle model a broad network of basal mantle ridges forms between 1000 and 600g€Ma, reflecting widely distributed subduction zones. Between 600 and 500g€Ma a short-lived degree-2 basal mantle structure forms in response to a band of subduction zones confined to low latitudes, generating extensive antipodal lower mantle upwellings centred at the poles. Subsequently, the northern basal structure migrates southward and evolves into a Pacific-centred upwelling, while the southern structure is dissected by subducting slabs, disintegrating into a network of ridges between 500 and 400g€Ma. From 400 to 200g€Ma, a stable Pacific-centred degree-1 convective planform emerges. It lacks an antipodal counterpart due to the closure of the Iapetus and Rheic oceans between Laurussia and Gondwana as well as due to coeval subduction between Baltica and Laurentia and around Siberia, populating the mantle with slabs until 320g€Ma when Pangea is assembled. A basal degree-2 structure forms subsequent to Pangea breakup, after the influence of previously subducted slabs in the African hemisphere on the lowermost mantle structure has faded away. This succession of mantle states is distinct from previously proposed mantle convection models. We show that the history of plume-related volcanism is consistent with deep plumes associated with evolving basal mantle structures. This Solid Earth Evolution Model for the last 1000 million years (SEEM1000) forms the foundation for a multitude of spatio-temporal data analysis approaches

Kinematic constraints on the Rodinia to Gondwana transition

Earth's plate tectonic history during the breakup of the supercontinent Pangea is well constrained from the seafloor spreading record, but evolving plate configurations during older supercontinent cycles are much less well understood. A relative paucity of available palaeomagnetic and geological data for deep time reconstructions necessitates innovative approaches to help discriminate between competing plate configurations. More difficult is tracing the journeys of individual continents during the amalgamation and breakup of supercontinents. Typically, deep-time reconstructions are built using absolute motions defined by palaeomagnetic data, and do not consider the kinematics of relative motions between plates, even for occasions where they are thought to be ‘plate-pairs’, either rifting apart leading to the formation of conjugate passive margins separated by a new ocean basin, or brought together by collision and orogenesis. Here, we use open-source software tools (GPlates/pyGPlates) to assess quantitative plate kinematics inherent within alternative reconstructions, such as rates of relative plate motion. We analyse the Rodinia-Gondwana transition during the Neoproterozoic, investigating the proposed Australia-Laurentia configurations during Rodinia, and the motion of India colliding with Gondwana. We find that earlier rifting times provide more optimal kinematic results. The AUSWUS and AUSMEX configurations with rifting at 800 Ma are the most kinematically supported configurations for Australia and Laurentia (average rates of 57 and 64 mm/a respectively), and angular rotation of ∼1.4°/Ma, compared to a SWEAT configuration (average spreading rate ∼76 mm/a) and Missing-Link configuration (∼90 mm/a). Later rifting, at, or after, 725 Ma necessitates unreasonably high spreading rates of >130 mm/a for AUSWUS and AUSMEX and ∼150 mm/a for SWEAT and Missing-Link. Using motion paths and convergence rates, we create a kinematically reasonable (convergence below 70 mm/a) tectonic model that is built upon a front-on collision of India with Gondwana, while also incorporating sinistral strike-slip motion against Australia and East Antarctica. We use this simple tectonic model to refine a global model for the breakup of western Rodinia and the transition to eastern Gondwana. © 2017 Elsevier B.V.This manuscript is a contribution to IGCP projects 628 (Gondwana Map) and 648 (Supercontinent Cycles and Global Geodynamics). This research was supported by the Science Industry Endowment Fund (RP 04-174) Big Data Knowledge Discovery Project, Australian Research Council grant DP130101946 (RDM) and the AuScope NCRIS project. ASM is supported by a CSIRO-Data61 Postgraduate Scholarship. ASC's contribution forms TRaX Record #379 and was funded by an Australian Research Council Future Fellowship FT120100340

Kinematic constraints on the Rodinia to Gondwana transition

Earth's plate tectonic history during the breakup of the supercontinent Pangea is well constrained from the seafloor spreading record, but evolving plate configurations during older supercontinent cycles are much less well understood. A relative paucity of available palaeomagnetic and geological data for deep time reconstructions necessitates innovative approaches to help discriminate between competing plate configurations. More difficult is tracing the journeys of individual continents during the amalgamation and breakup of supercontinents. Typically, deep-time reconstructions are built using absolute motions defined by palaeomagnetic data, and do not consider the kinematics of relative motions between plates, even for occasions where they are thought to be ‘plate-pairs’, either rifting apart leading to the formation of conjugate passive margins separated by a new ocean basin, or brought together by collision and orogenesis. Here, we use open-source software tools (GPlates/pyGPlates) to assess quantitative plate kinematics inherent within alternative reconstructions, such as rates of relative plate motion. We analyse the Rodinia-Gondwana transition during the Neoproterozoic, investigating the proposed Australia-Laurentia configurations during Rodinia, and the motion of India colliding with Gondwana. We find that earlier rifting times provide more optimal kinematic results. The AUSWUS and AUSMEX configurations with rifting at 800 Ma are the most kinematically supported configurations for Australia and Laurentia (average rates of 57 and 64 mm/a respectively), and angular rotation of ∼1.4°/Ma, compared to a SWEAT configuration (average spreading rate ∼76 mm/a) and Missing-Link configuration (∼90 mm/a). Later rifting, at, or after, 725 Ma necessitates unreasonably high spreading rates of >130 mm/a for AUSWUS and AUSMEX and ∼150 mm/a for SWEAT and Missing-Link. Using motion paths and convergence rates, we create a kinematically reasonable (convergence below 70 mm/a) tectonic model that is built upon a front-on collision of India with Gondwana, while also incorporating sinistral strike-slip motion against Australia and East Antarctica. We use this simple tectonic model to refine a global model for the breakup of western Rodinia and the transition to eastern Gondwana. © 2017 Elsevier B.V.This manuscript is a contribution to IGCP projects 628 (Gondwana Map) and 648 (Supercontinent Cycles and Global Geodynamics). This research was supported by the Science Industry Endowment Fund (RP 04-174) Big Data Knowledge Discovery Project, Australian Research Council grant DP130101946 (RDM) and the AuScope NCRIS project. ASM is supported by a CSIRO-Data61 Postgraduate Scholarship. ASC's contribution forms TRaX Record #379 and was funded by an Australian Research Council Future Fellowship FT120100340

Dynamic redox and nutrient cycling response to climate forcing in the Mesoproterozoic ocean

Controls on Mesoproterozoic ocean redox heterogeneity, and links to nutrient cycling and oxygenation feedbacks, remain poorly resolved. Here, we report ocean redox and phosphorus cycling across two high-resolution sections from the ~1.4 Ga Xiamaling Formation, North China Craton. In the lower section, fluctuations in trade wind intensity regulated the spatial extent of a ferruginous oxygen minimum zone, promoting phosphorus drawdown and persistent oligotrophic conditions. In the upper section, high but variable continental chemical weathering rates led to periodic fluctuations between highly and weakly euxinic conditions, promoting phosphorus recycling and persistent eutrophication. Biogeochemical modeling demonstrates how changes in geographical location relative to global atmospheric circulation cells could have driven these temporal changes in regional ocean biogeochemistry. Our approach suggests that much of the ocean redox heterogeneity apparent in the Mesoproterozoic record can be explained by climate forcing at individual locations, rather than specific events or step-changes in global oceanic redox conditions

A tectonic-rules-based mantle reference frame since 1 billion years ago – implications for supercontinent cycles and plate–mantle system evolution

Understanding the long-term evolution of Earth's plate–mantle system is reliant on absolute plate motion models in a mantle reference frame, but such models are both difficult to construct and controversial. We present a tectonic-rules-based optimization approach to construct a plate motion model in a mantle reference frame covering the last billion years and use it as a constraint for mantle flow models. Our plate motion model results in net lithospheric rotation consistently below 0.25∘ Myr−1, in agreement with mantle flow models, while trench motions are confined to a relatively narrow range of −2 to +2 cm yr−1 since 320 Ma, during Pangea stability and dispersal. In contrast, the period from 600 to 320 Ma, nicknamed the “zippy tricentenary” here, displays twice the trench motion scatter compared to more recent times, reflecting a predominance of short and highly mobile subduction zones. Our model supports an orthoversion evolution from Rodinia to Pangea with Pangea offset approximately 90∘ eastwards relative to Rodinia – this is the opposite sense of motion compared to a previous orthoversion hypothesis based on paleomagnetic data. In our coupled plate–mantle model a broad network of basal mantle ridges forms between 1000 and 600 Ma, reflecting widely distributed subduction zones. Between 600 and 500 Ma a short-lived degree-2 basal mantle structure forms in response to a band of subduction zones confined to low latitudes, generating extensive antipodal lower mantle upwellings centred at the poles. Subsequently, the northern basal structure migrates southward and evolves into a Pacific-centred upwelling, while the southern structure is dissected by subducting slabs, disintegrating into a network of ridges between 500 and 400 Ma. From 400 to 200 Ma, a stable Pacific-centred degree-1 convective planform emerges. It lacks an antipodal counterpart due to the closure of the Iapetus and Rheic oceans between Laurussia and Gondwana as well as due to coeval subduction between Baltica and Laurentia and around Siberia, populating the mantle with slabs until 320 Ma when Pangea is assembled. A basal degree-2 structure forms subsequent to Pangea breakup, after the influence of previously subducted slabs in the African hemisphere on the lowermost mantle structure has faded away. This succession of mantle states is distinct from previously proposed mantle convection models. We show that the history of plume-related volcanism is consistent with deep plumes associated with evolving basal mantle structures. This Solid Earth Evolution Model for the last 1000 million years (SEEM1000) forms the foundation for a multitude of spatio-temporal data analysis approaches

Deconvolving the pre-Himalayan Indian margin – tales of crustal growth and destruction

The metamorphic core of the Himalaya is composed of Indian cratonic rocks with two distinct crustal affinities that are defined by radiogenic isotopic geochemistry and detrital zircon age spectra. One is derived predominantly from the Paleoproterozoic and Archean rocks of the Indian cratonic interior and is either represented as metamorphosed sedimentary rocks of the Lesser Himalayan Sequence (LHS) or as slices of the distal cratonic margin. The other is the Greater Himalayan Sequence (GHS) whose provenance is less clear and has an enigmatic affinity. Here we present new detrital zircon Hf analyses from LHS and GHS samples spanning over 1000 kilometers along the orogen that respectively show a striking similarity in age spectra and Hf isotope ratios. Within the GHS, the zircon age populations at 2800–2500 Ma, 1800 Ma, 1000 Ma and 500 Ma can be ascribed to various Gondwanan source regions; however, a pervasive and dominant Tonian age population (∼860–800 Ma) with a variably enriched radiogenic Hf isotope signature (εHf = 10 to -20) has not been identified from Gondwana or peripheral accreted terranes. We suggest this detrital zircon age population was derived from a crustal province that was subsequently removed by tectonic erosion. Substantial geologic evidence exists from previous studies across the Himalaya supporting the Cambro-Ordovician Kurgiakh Orogeny. We propose the tectonic removal of Tonian lithosphere occurred prior to or during this Cambro-Ordovician episode of orogenesis in a similar scenario as is seen in the modern Andean and Indonesian orogenies, wherein tectonic processes have removed significant portions of the continental lithosphere in a relatively short amount of time. This model described herein of the pre-Himalayan northern margin of Greater India highlights the paucity of the geologic record associated with the growth of continental crust. Although the continental crust is the archive of Earth history, it is vital to recognize the ways in which preservation bias and destruction of continental crust informs geologic models

Transient mobilization of subcrustal carbon coincident with Palaeocene–Eocene Thermal Maximum

Plume magmatism and continental breakup led to the opening of the northeast Atlantic Ocean during the globally warm early Cenozoic. This warmth culminated in a transient (170 thousand year, kyr) hyperthermal event associated with a large, if poorly constrained, emission of carbon called the Palaeocene–Eocene Thermal Maximum (PETM) 56 million years ago (Ma). Methane from hydrothermal vents in the coeval North Atlantic Igneous Province (NAIP) has been proposed as the trigger, though isotopic constraints from deep sea sediments have instead implicated direct volcanic carbon dioxide (CO2) emissions. Here we calculate that background levels of volcanic outgassing from mid-ocean ridges and large igneous provinces yield only one-fifth of the carbon required to trigger the hyperthermal. However, geochemical analyses of volcanic sequences spanning the rift-to-drift phase of the NAIP indicate a sudden ~220 kyr-long intensification of magmatic activity coincident with the PETM. This was likely driven by thinning and enhanced decompression melting of the sub-continental lithospheric mantle, which critically contained a high proportion of carbon-rich metasomatic carbonates. Melting models and coupled tectonic–geochemical simulations indicate that >104 gigatons of subcrustal carbon was mobilized into the ocean and atmosphere sufficiently rapidly to explain the scale and pace of the PETM